Wed 25 May 2005
By markmc misc

Debugging With Strace

I just check in a short doc explaining some tricks to use when debugging Vino.

I got a bit side tracked explaining how you can figure out what messages are being sent back and forth to the X server, just by looking at the read()s and write()s in the strace. A snippet:

[markmc@blaa ~]$ grep -rn X_QueryExtension /usr/include/X11/Xproto.h
2091:#define X_QueryExtension               98

  That's 0x62 in hex. So, we're looking for a write to the X
connection (file descriptor 3) with 0x62 as the first byte. What do ya
know:

1117012824.950683 writev(3, [{"\x62\x01\x04\x00\x06\x00\x01\x00", 8}, {"DAMAGE", 6}, {"\x00\x00", 2}], 3) = 16
1117012824.950939 read(3, "\x01\x00\x52\x00\x00\x00\x00\x00\x01\x9d\x75\xba\x00\x00"..., 32) = 32

  It's pretty clear that its QueryExtension for DAMAGE. Now, lets
figure out the event base from the reply. The format for the reply is:

typedef struct {
    BYTE type;  /* X_Reply */
    BYTE pad1;
    CARD16 sequenceNumber B16;
    CARD32 length B32; /* 0 */
    BOOL  present;
    CARD8 major_opcode;
    CARD8 first_event;
    ...
} xQueryExtensionReply;

   first_event is 11 bytes in. Looking at the read, that's 0x75. The
value of XDamageNotify is zero, so we can be 100% sure that all those
events after our NoExpose events are XDamageNotify events.

Some people might find it interesting. Others will think I'm weird and tell me to use something like xscope.

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